1. Field of the Invention
The invention is related to the growth of Group-III nitride crystals, and more particularly, to the growth of Group-III nitride crystals in supercritical ammonia using an autoclave.
2. Description of the Related Art
(Note: This application references a number of different publications and patents as indicated throughout the specification by one or more reference numbers within brackets, e.g., [x]. A list of these different publications and patents ordered according to these reference numbers can be found below in the section entitled “References.” Each of these publications and patents is incorporated by reference herein.)
The usefulness of gallium nitride (GaN) and its ternary and quaternary alloys incorporating aluminum and indium (AlGaN, InGaN, AlInGaN) has been well established for fabrication of visible and ultraviolet optoelectronic devices and high-power electronic devices. These devices are typically grown epitaxially on heterogeneous substrates, such as sapphire and silicon carbide since GaN wafers are not yet available. The heteroepitaxial growth of group III-nitride causes highly defected or even cracked films, which deteriorate the performance and reliability of these devices.
In order to eliminate the problems arising from the heteroepitaxial growth, group III-nitride wafers sliced from bulk crystals must be used. However, it is very difficult to grow a bulk crystal of group III-nitride, such as GaN, AlN, and InN, since group III-nitride has a high melting point and high nitrogen vapor pressure at high temperature.
Up to now, a couple of methods, such as high-pressure high-temperature synthesis [1,2] and sodium flux [3,4], have been used to obtain bulk group III-nitride crystals. However, the crystal shape obtained by these methods is a thin platelet because these methods are based on a melt of group III metal, in which nitrogen has very low solubility and a low diffusion coefficient.
A new technique called ammonothermal growth has the potential for growing large bulk group III-nitride crystals, because supercritical ammonia used as a fluid has high solubility of source materials, such as group III-nitride polycrystals or group III metal, and has high transport speed of dissolved precursors. This ammonothermal method [5-9] has a potential of growing large bulk group III-nitride crystals.
However, in the previously disclosed technique, there was no quantitative assessment for the grain size of the source material. If GaN or AlN is chosen as a source material, the only commercially available form is a powder of a size less than 10 microns, and usually 0.1˜1 microns. This small powder is easily blown by the convective flow of supercritical ammonia and transported onto the seed crystals, resulting in polycrystalline growth.
The main idea of the ammonothermal growth is taken from a successful mass production of artificial quartz by hydrothermal growth. In the hydrothermal growth of artificial quartz, an autoclave is divided into two regions: a top region and a bottom region. Source material, known as the nutrient, such as polycrystalline SiO2, is placed in the bottom region and seed crystals, such as single crystalline SiO2, are placed in the top region. The autoclave is filled with water and a small amount of chemicals known as mineralizers are added to the water to increase the solubility of SiO2. Sodium hydroxide or sodium carbonate is a typical mineralizer. In addition, the temperature in the bottom region is kept higher than that in the top region.
In the case of ammonothermal growth, ammonia is used as a fluid. It is challenging to fill the autoclave with liquid ammonia safely, without contamination. In particular, oxygen is a detrimental impurity source in ammonothermal growth. Both the ammonia and mineralizers favor oxygen and moisture. Therefore, it is very important to load all solid sources and ammonia in an air-free environment.
Another important issue is the boiling point of ammonia. In the case of hydrothermal growth, water is in a liquid phase at room temperature. However, ammonia is in a gas phase at room temperature and the vapor pressure at room temperature is about 150 psi. It is necessary to cool the autoclave and condense gaseous ammonia to fill liquid ammonia into an autoclave or an internal chamber.
When the size of the autoclave is small (e.g., small enough to fit in a glove-box), all solid sources (i.e., nutrient, mineralizers, seed crystals, etc.) can be loaded into the autoclave in a glovebox, and ammonia can be condensed in the autoclave by cooling the entire autoclave. However, when the autoclave is large (e.g., too large to fit in a glove-box), it is practically very difficult to cool the entire autoclave to condense the ammonia.
These difficulties can be solved by using an internal chamber within the autoclave. However, use of an internal chamber creates another problem, which is to balance pressure inside and outside of the internal chamber.
Notwithstanding the above, what is needed in the art are new methods for the growth of group-III nitride structures, as well as new apparatus for performing such methods. The present invention satisfies these needs.